Positron Production from a Tungsten Single Crystal
at the KEK 8-GeV Electron LinacK.Sasahara and R.Hamatsu
Graduate School of Science, Department of Physics, Tokyo Metropolitan University, 1-1 MinamiOhsawa, Hachioji, Tokyo, 192-0397, Japan
S.Anami, A.Enomoto, K.Furukawa, K.Kakihara, T.Kamitani, Y.Ogawa, A.Ohsawa, T.Oogoe, andT.Suwada
Accelerator Laboratory, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba
Ibaraki, 305-0801,JapanH.Okuno
Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK),
1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
T.Fujita, K.Umemori, and K.Yoshida
Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, HigashiHiroshima 739-8526, Japan
V. Ababiy, A.P.Potylitsyn, I.E.Vnukov
Nuclear Physics Institute, Tomsk Polytechnic University, P.O.Box 25 Lenin Ave. Tomsk, 604050
Russian Federation
In order to study a possibility of using a tungsten (W) single crystal as a high-intensity positron sourse at
linear accelerators, we carried out an experiment to measure the positron production efficiency by the 8GeV beam from the KEK electron linac. The single-bunch electron beam with a bunch width of 10 ps, 2 Hz
repetition, and 0.2 nC/bunch intensity was injected along the <111> axis of the W single crystal mounted
on a precision goniometer. Positrons were measured with a magnetic spectrometer placed downstream fromthe target at 0 degree with respect to the beam line. When the <111> axis of the crystal was aligned to theelectron beam, about 5.1 times enhancement in the 2.2mm-thick crystal was observed compared to thepositron yield for the disoriented crystal at Pe+ = 20 MeV/c and about 1.7 times in the 9mm-thick one.

I. INTRODUCTION

For B-factories and future electron-positron linear
colliders, a high-intensity positron source is very
important in executing a high-luminosity beamcollision. In a general way, positrons for accelerators areproduced by the electromagnetic cascade shower whenhigh-energy electrons hit a heavy-metal amorphoustarget such as tungsten (W). For the subsequent
acceleration, the momentum and angle are restricted tothe small range. For example, in the KEKB positronstation, the acceptance for produced positrons is Pt ≤ 2.4
MeV/c, and 8.2 MeV/c ≤ P ≤11.6 MeV/c. The thicknessof W amorphous target is 14mm (4Xo), which is theoptimum thickness for the 4-GeV incident electron
beam. In this case, the number of captured positrons is
at most about 0.1 positrons/electron. In such a
conventional source, the positron intensity is limited byincident beam power capability and a heat load on thetarget. Then, one of the promising methods is to use asingle crystal as a target for positron generation toincrease the positron production efficiency.
The physical background of this method has tworadiation processes reflecting the character of a crystal.
One is the coherent bremsstrahlung. Another is thechanneling radiation. When the high-energy electrons
are injected into a crystal almost in parallel with its axis,
electrons are captured by the axial potential of a crystal,move spirally (axial channeling), and then emit
channeling radiation. When the photons from theseradiation processes cause pair creation, the highintensity positrons are obtained. These radiations havethe advantage for production of positrons withcomparatively low momentum, which match to theacceptance of the later acceleration. [1]The possibility of a target for positron generation
using the channeling radiation from a single crystal wasproposed by Chehab et al. in 1989, and the simulation
was performed. [2] Using the 1.2 mm-thick W single
crystal, our group made a 'proof of principle' experimentby the 1.2-GeV electron synchrotron at the KEK
Tanashi branch in 1996. When the crystal axis wasaligned to the electron beam, 2.6 times enhancement
was observed compared to the positron yield for thedisoriented crystal at Pe+ = 20 MeV/c. [3] Then, at theKEK 3-GeV Linac, an experiment was done with thetarget which combined a 1.7mm W single crystal and7mm W amorphous, and 1.4 times enhancement wasobserved in 1998. [4] In 2000, using a 8-GeV shortbunch electron beam from the KEK Linac, we carriedout the experiment and observed about 5 times
enhancement in the 2.2 mm W crystal, and 1.2-1.9 times
enhancement in combination of 2.2 mm crystal and 5,
10 mm amorphous. [5] In the same year, Chehab et al.
have begun the experiment with a electron beam of 5-40
GeV in CERN-SPS. [6] In April 2001, we measured thepositron generation efficiency using W single crystals,
whose thickness is 2.2 mm and 9 mm, at the KEK 8GeV Linac. These systematic experiments are importantfor investigating the possibility of using a single crystal
as a positron source in accelerators.

electron beam was injected along the direction of the<111> axis. In the case of the amorphous, the target
was mounted on X-Stage installed behind thegoniometer, which can move in the X direction(perpendicular to the beam axis), and the data wastaken by changing the thickness by moving the target in
the X direction.

Fig.1. Photograph of the experimental setup.

II. Experimental Method and Apparatus
The experiment was carried out with the test beamline of the beam switchyard at the KEKB injector Linac.
The rear view of the setup is shown in Fig. 1.
Parameters of the incident electron beam were;
Energy = 8 GeV
Intensity = 0.2 nC/bunch
Bunch width = 10 ps
Repetition rate = 2 Hz.
The incident electron beam was taken out in theatmosphere through a 100 μ m SUS window, andinjected to the target mounted on a goniometer. As
shown in the right side of Fig. 2, the goniometer canmove four axes of X, V, H, and A by stepping motors,
and can change the angle of the target precisely withrespect to the beam direction. The beam size at thetarget was about 1mm. The beam divergence was 15μ
rad in the transverce direction and 72 μ rad in thehorizontal direction. We used the W crystal targets of
2.2 mm (mosicity ~1.5mrad) and 9 mm (mosicity~0.5mrad). Amorphous W targets with different
thickness from 3 to 18 mm by 3mm steps were also used
for comparison. In the case of crystal targets, theFig. 2. Outline figure of the experiment
As shown in the left side of Fig. 2, the positrons
generated from the target passed collimators of 30 mm
φ and 40 mmφ installed in the vacuum chamber, andwere bent 60 degrees from a beam axis with an analyzer
magnet. An analyzer magnet current was set to selectpositrons in the momentum range between 10 and 20
MeV/c. Momentum selected positrons passed threecollimators of 20 mmφ were detected by the positrondetectors, an acrylic Cherenkov counter and a lead-glass
calorimeter. Pulse heights of these counters wererecorded bunch by bunch togeter with the beams
intensity values, since the bunch width from the linac
was 10 ps, within which positrons could not be resolved
by detectors. Beam currents were measured by a wallcurrent monitor and recorded at the same time. Pedestals
and backgrounds were measured frequently during thecource of experiments and subtracted from the signal
data.

III. EXPERIMENTAL RESULTS

The positron yields at Pe+ = 20MeV/c as a function ofthe goniometer angle were shown for W single crystalsof 2.2 mm and 9 mm thick in Fig.3. (Rocking curve)
When the relative positron yields at Pe+ = 20MeV/c
were compared at On-axis and Off-axis goniometer
angles, 5.1 ± 0.1 and 1.7 ± 0.1 times enhancements for2.2 mm and 9 mm-thick crystals were observed
respectively. Angular widths of peaks were obtained byfitting to a Lorentzian function to be 9.0 mrad and 39.2mrad FWHM for 2.2mm and 9mm thick crystals
respectively.
The enhancements as a function of the positron
momentum are shown in Fig.4. The figure indicates thatthe enhancement has a tendency to increase as themomentum becomes lower.
We also tested the amorphous target in the sameway. For reference, Fig.5 illustrates a comparison of thepositron yield of W crystal target with amorphous one.
The vertical axis is the relative positron yield correctedby the acceptance for different amorphous targets.
Assuming that the yields at Off-axis for the crystal
targets are the same as for the amorphous withcorresponding thickness, positron yields for crystal wereplotted in the figure by multiplying the enhancement
factors to the amorphous yields.

Fig.3. Rocking curves for W single crystal at
Pe+=20MeV/c, Ee-=8GeV. (a) 2.2mm-thick W
crystal target. (b) 9mm-thick W crystal target. The
curves are fits to a Lorentzian function plus a
constant yield.

IV. SUMMARY

We conducted a positron production experiment froma W single crystal as a target in 8-GeV energy region
using the electron beam from the KEKB injector linac.
When the <111> axis of the crystal was aligned to theelectron beam, 5.1 and 1.7 times more positrons than thedisoriented crystal were observed at Pe+ = 20 MeV/c for
2.2 mm and 9 mm thickness respectively. These rocking
curves show that peak widths increase as a crystal
becomes thick. These angluar widths are very large
when compared with the channeling critical angle at the8-GeV electron beam; 0.43mrad (the Lindhart angle),
and the multiple scattering angles; 1.34 mrad (in2.2mm) and 2.78 mrad (in 9mm). We are stillFig.4. Momentum dependence of the positronenhancement.

By comparing the positron yields from the crystal
and amorphous targets, it is noted that the much thinner
crystal target can be used than the amorphous target fora given amount of positrons. It is remarkable that thepositron yield of the 9 mm-thick W crystal iscomparable to that of 14 mm-thick W amorphous,which is used at the positron station of the KEKB linac.
There are possibilities to reduce the multiple scatteringand a heat load in the target by making a target thinner.

REFERENCESFig.5 . Relative positron yield from W crystal andW amorphous. Errors are statistical only.

investigating the possible cause of the increase ofangular width of the rocking curve.
About the momentum dependence of theenhancements, the enhancement increases as themomentum becomes lower. This is advantageous for theacceptance of the subsequent acceleration of positrons at
the injector linac.

[1] S.Anami, et al., KEK Preprint 2000-138, 2001
[2] R.Chehab, et al., Orsay Report LAL-RT 89-01, 1988
[3] K.Yoshida, et al., Phys. Rev. Lett. 80, 1437, 1998
[4] M.Inoue et al, NIM B 173, p.104, 2001.
[5] T.Suwada, et al., Proceedings of HEACC2001,
Tsukuba, Japan, Mar.2001[6] R.Chehab, et al., Proceedings of LINAC2000,
Monterey, CA, USA, p.143, 2000